Weight is an important factor in aircraft design. Using lighter components is one way of reducing weight in an aircraft.
Wiring is an important component in every aircraft, and contributes significantly to the weight of an aircraft. In this context, a wire may be understood to include a conductor provided with a protective outer layer referred to as insulation. Furthermore, the term "wiring" as used herein may be understood to include conductive wires that carry signals, current, and the like. The term "testing" of wiring as used herein may be understood to include testing various qualities of the conductor, the insulation, or both.
Wires may be bundled together and enclosed with a common shield made of various materials such as nylon, rubber, and the like. On the other hand, it is frequently the case that a bundle of wires is not enclosed with a common shield. In either case, however, such a bundle of wires may be referred to as a cable. Other common terms for a cable are a harness or a loom.
There are various approaches to making the wiring in an aircraft design weigh less. One approach is to reduce the length of wire as much as possible. Another approach is to use the smallest conductor that is considered to be safe for the load. Yet another approach is to equip the wiring with a lightweight insulation.
Although the use of lightweight insulation is a prominent approach to reducing the overall weight of the wiring component in an aircraft design, this approach includes risks not heretofore appreciated. One such risk is the risk of degradation of the insulation. Degradation may result from vibration, chafing, or impact. Degradation may result from the simple aging of the insulation and from the exposure of the insulation to harmful environments. As used here, degradation means damage to insulation from any source whatever.
The insulation is an important part of the wiring because it keeps conductors of different wires from undesirably coming into contact with each other and from undesirably coming into contact with an electrical ground. When different wires undesirably come into contact with each other, short circuits can occur. Insulation also keeps the conductor of a wire from undesirably coming into contact with the environment surrounding the wire. When the conductor of a wire undesirably comes into contact with its surrounding environment, sparks, fires, and explosions can result. Another possible result of such an undesirable contact is the malfunction of the connected or nearby equipment, or the propagation of invalid data or unwanted noise.
This invention relates to the testing of wires for degradation. The degradation tested is primarily degradation of the insulation. Even more particularly, this invention relates to the testing of wiring in situ, or in its normal place of operation. This invention was developed in the particular context of aircraft wiring, and the examples described herein are given in that context. Nevertheless, the invention is extremely useful for testing wires for degradation in situ in any setting. In particular, it is envisioned that the invention may be used with wiring installed in such settings as the home, in ships, underground, in buildings, or the like.
There are certain problems with the testing of in-place wiring insulation.
First of all, a cable may contain numerous wires, each with its own insulation that needs to be tested.
A second problem is that the wires to be tested may be inaccessible except at the ends. In an aircraft, or in an underground wiring system, cables containing wires may extend for hundreds of yards, or even miles, without the opportunity for access. Ideally, a test that identifies wiring degradation should take inaccessibility into account. Moreover, it is very often the case that only one of the two ends is available or accessible. Where only one end is accessible, a loop-back type of test is substantially impossible. A test that accounts for single-end accessibility is required.
A third problem with testing wires for degradation is that some testing methods may actually cause damage to the insulation, or to the conductor, or to the connected equipment. For example, testing a wire using high voltage may damage equipment connected to the other end of the wire, or may even further injure degraded insulation.
The following patents are representative of the art, and each is incorporated by reference in its entirety for its useful background information: U.S. Pat. Nos. 464,125; 2,526,891; 2,942,181; 3,349,324; 3,820,011; and 4,891,597.
In U.S. Pat. No. 464,125, there is disclosed a technique for testing insulated wires. In particular, a bundle of insulated wire is immersed in a tank of water. An electromotive force is provided at one end of the wire at the conductor of the wire. A conductor is placed in the water. The insulation on the wire is determined to be defective if a current can be induced to flow from the one end of the wire connected to the electromotive force, through the insulation, and to the conductor that is in the tank of water.
This method and apparatus for testing wire insulation is unsuitable for use in aircraft or the like because it requires the complete removal of the wire or cable, and its immersion in a big vat of water. Big vats of water cannot reasonably be used in an aircraft or the like in view of the risk of harmful effects to the surrounding environment. Another reason that big vats of water cannot reasonably be used for in situ testing is that wires are often affixed to elevated parts of the interior structure, and it is very hard to remove the wire far enough from the interior so as to submerge it in such a vat.
In U.S. Pat. No. 2,526,891, there is disclosed a method of testing electrical conductors. According to this method, a twisted pair of wires is tested by applying a DC polarizing potential across the two wires for a period of time sufficient to create electrolytic polarization in the moisture of the sheaths, disconnecting the source of polarizing potential from the wires, and measuring the electrolytic polarization potential residual in the moisture after disconnection of the polarizing potential.
According to this approach, the wire must be a twisted pair of copper wires each sheathed with a seamless sheath of paper pulp. It is assumed that the paper pulp sheaths already contain as much as 15 percent water weight, either residual from the making of the sheaths or outdoor from the atmosphere during storage. The teachings of this patent relate to the manufacturing stage of wire and, more particularly, to the detection of breaks in a wire when the amount of moisture in the paper pulp sheaths is enough to cause a false reading that the conductor is not broken. No conductive medium is applied to the wires, and there must be at least two breaks in the insulation for a detection to be made.
U.S. Pat. No. 2,942,181 describes an apparatus for testing cables. This cable-testing apparatus tests a cable as it is wound from one reel to another reel. As the cable moves between the two reels, it is immersed in a tank of water. One end of the cable is connected to an electromotive force. Submerged in the tank of water is. an electrode. If the electromotive force can cause a current to flow from one end of the cable through a crack in the insulation and to the electrode submerged in the water, then a defect in the cable is detected.
This approach is unsuitable for use where the wires to be tested cannot be removed and dipped in the tank of water. Furthermore, this United States patent does not contain any teaching or suggestion relating to how much electromotive force should be used in the attempt to cause a current to flow to the submerged electrode. In addition, in the foregoing approach assumes that there is no sensitive or delicate equipment connected to the cable.
U.S. Pat. No. 3,349,324 describes an apparatus for locating faults in insulated electric cables. The apparatus has an elongated tube that contains a conducting liquid electrode. According to this approach, a cable is transferred from one reel to another. During the transfer, the cable passes through a tube filled with water. The tube is pressurized. The tube itself is made of an electrical conducting material. Under this approach, an electromotive force is applied to the tube itself, while one end of the cable is grounded.
With the foregoing approach, it is necessary that the entire tube be insulated from the ground. Also, it is assumed that the cable is completely disconnected from any sensitive or delicate equipment, and also that the cable is not installed, but is passed from one reel to another. This method is therefore entirely unsuitable for use in an in situ test.
U.S. Pat. No. 3,820,011 provides for a trough into which a wire is submerged so that faults in the insulation around the wire can be detected. The testing described in this document describes the use of low voltage in the detection of faults. Under this approach, however, it is assumed that the wire is fed through the trough. Thus, the wire must be in a state in which it is not installed in a restrictive area, such as an aircraft or the like.
U.S. Pat. No. 5,206,597 describes a capacitive moisture detection apparatus. In particular, the apparatus is meant to measure the volume of water intrusion in underwater cables without having to take the cables apart. According to this method, a cable is fed from one reel to another through an elongated tube. In the elongated tube, which is a cylindrical brass shell, there are two curved electrical conducting plates which serve as capacitance plates. The volume of water that has infiltrated the cable is determined by measuring the capacitance.
None of these patents provides for an acceptable in situ testing of a wire for degradation, and none provides for any protection of the equipment that remains connected to the cable during the test.
Furthermore, in situ testing of a wire in an aircraft or the like often involves wires that are suspended overhead or are affixed or attached to walls of the aircraft.